This invention relates to a converter including a fly-back circuit.
The converter according to the invention is preferably used as a part of a switching power supply in entertainment electronics devices.
U.S. Pat. No. 4,683,529 describes a switching power supply including a fly-back circuit and a control circuit for power factor correction. The switching power supply converts a line voltage into a regulated DC output voltage and is adapted for operation with different line voltages and line voltage frequencies which vary internationally. The control circuit controls the switching transistor of the fly-back circuit by means of a pulse width modulated control signal having a suitable frequency, so that the fly-back circuit operates on the borderline between continuous and discontinuous operation to minimize the switching losses. Furthermore, the switching power supply provides a high power factor under various mains and load conditions.
It is an object of the invention to provide a converter including a fly-back circuit (i.e. high frequency boost converter) arranged so that an optimum adaptation to the internationally different line voltages is achieved.
This object is achieved in that with a converter input voltage that lies in a first voltage range a first fly-back switching frequency range is provided and in that with a converter input voltage lying in a second voltage range a second fly-back switching frequency range is provided, the first voltage range having lower voltage values than the second voltage range and the first fly-back switching frequency range having higher frequency values than the second fly-back switching frequency range.
In this way the internationally different EMI requirements (Electro Magnetic Interference) in Europe and the USA for entertainment electronics devices are satisfied by a respectively adapted selection of the fly-back switching frequency ranges. This arrangement leads to switching power supplies that have small dimensions and little weight.
The inventive idea is more particularly realized in that the converter operates discontinuously in the second voltage range and in that approximately the same top limit for the current is provided by the fly-back inductance both during operation in the first voltage range and during operation in the second voltage range. More particularly, the converter operates in the boundary region between continuous and discontinuous operation in the first voltage range and under full load, whereas the converter operates discontinuously in the first voltage range and under partial load, and the converter operates discontinuously in the second voltage range both under fall load and under partial load.
In another embodiment of the converter according to the invention a first capacitor supplies the supply voltage of an integrated switching circuit which is used for supplying the control signal for the fly-back switching element and the first capacitor is coupled to a node between the fly-back inductance, fly-back switching element and fly-back diode via a second capacitor and a further diode via which the first capacitor is charged in the manner of a charge pump. This measure guarantees the voltage supply of the integrated circuit via simple circuit modifications.
In another circuit variant a circuit block is connected to the node between the fly-back inductance, the fly-back switching element and the fly-back diode and produces a signal for the recognition of the zero state of the current through the fly-back diode and/or the state of the minimum voltage on the switch. The signal thus supplied can be used for determining the optimum instant for turning on the fly-back switching element, so that the switching losses are minimized.
The transition from the first to the second operating mode (i.e. first voltage range to second voltage range) is preferably effected continuously.